1. Field of the Invention
The present invention relates to a valve timing control system, which adjusts opening and closing timing (hereinafter, referred to as the valve timing) of at least one of an intake valve and an exhaust valve of an internal combustion engine.
2. Description of Related Art
For example, as recited in Japanese Unexamined Patent Publication No. 2006-46315 (corresponding to U.S. Pat. No. 7,182,052), a previously proposed valve timing control system includes a housing and a vane rotor. The housing receives a drive force of a crankshaft of the internal combustion engine, and the vane rotor is received in the housing and transmits the drive force of the crankshaft to the camshaft. The valve timing control system employs the pressure of a working fluid in a retard chamber and an advance chamber to drive the vane rotor to rotate toward the retard side or the advance side relative to the housing. In this way, the phase of the camshaft relative to the crankshaft is adjusted, i.e., the valve timing is adjusted.
When the intake valve or the exhaust valve is opened or closed through such a valve timing control system, the torque fluctuation, which is received by the camshaft from the intake valve or exhaust valve, is conducted to the vane rotor. Thus, the vane rotor receives the torque fluctuation toward the retard side and the advance side relative to the housing.
For example, in the case where the working fluid is supplied to the advance chamber to change the phase of the camshaft relative to the crankshaft from the retard side to a target phase on the advance side, when the vane rotor receives the torque fluctuation toward the retard side, the vane rotor receives the torque fluctuation in the direction for causing a reduction in the volume of the advance chamber. Thus, the working fluid in the advance chamber receives the force that causes discharge of the hydraulic fluid from the advance chamber. Then, as shown by a dotted line in FIG. 15, the vane rotor is returned toward the retard side due to the torque fluctuation. Thus, a response time period, which is required to reach the target phase, is lengthened. This disadvantageous phenomenon becomes particularly prominent when the pressure of the working fluid, which is supplied from a fluid source, is relatively low.
In view of this, as recited in Japanese Unexamined Patent Publication No. 2006-46315, it is conceivable to provide a check valve in a supply passage, which supplies the working fluid to the advance chamber, to limit the discharge of the working fluid from the advance chamber even when the vane rotor receives the torque fluctuation. In this way, as indicated by a solid line in FIG. 15, it is possible to limit the returning of the vane rotor to the side opposite from the target phase relative to the housing in the phase control operation, and thereby the response in the phase control operation can be improved.
Also, in Japanese Unexamined Patent Publication No. 2006-46315, a control valve is provided in a bypass discharge passage that connects between the advance chamber and the fluid source while bypassing the check valve, which is installed in the supply passage that supplies the working fluid to the advance chamber. For example, the control valve may be a switch valve, which blocks the bypass discharge passage when the switch valve receives the pressure of the working fluid supplied to the advance chamber or the retard chamber as a pilot pressure. When the pilot pressure is not applied to the switch valve, the switch valve opens the bypass discharge passage. That is, the control valve blocks the bypass discharge passage when the advance control operation is executed to supply the hydraulic fluid to the advance chamber. Thus, it is possible to limit the discharging of the working fluid from the advance chamber to the bypass discharge passage while bypassing the check valve during the advance control operation. In contrast, when the retard control operation is executed to discharge the hydraulic fluid from the advance chamber, the control valve opens the bypass discharge passage. Thus, the working fluid is discharged from the advance chamber to the bypass discharge passage while bypassing the check valve.
However, according to Japanese Unexamined Patent Publication No. 2006-46315, the pressure of the hydraulic fluid in the passage connected to the advance chamber or the retard chamber is received as a pilot pressure. Here, the passage supplies the hydraulic fluid to the advance chamber or the retard chamber and discharges the hydraulic fluid from the advance chamber or the retard chamber. Thus, when the vane rotor receives the torque fluctuation during the phase control operation to cause the fluctuation of the pressure of the advance chamber or of the retard chamber, pressure pulsation may possibly occur in the pilot pressure applied to the control valve. When the pressure pulsation occurs in the pilot pressure, the control valve may not be appropriately switched by using the pilot pressure.